Figure 1. Axial T₁ weighted MRI images of a SCID mouse bearing an MCF-7 human breast cancer xenograft in the flank (arrow): (a) pre-Magnevist®, (b) 10 min post-contrast, (c) 20 min post-contrast and (d) 30 min post-contrast. The heterogeneity of enhancement is indicative of non-uniform wash-in and wash-out kinetics of the contrast agent. Such heterogeneity is routinely observed in both animal and human tumours, and illustrates tumour perfusion heterogeneity.